Visual Cell Locator

This is a small device that you wear like a wristwatch (so you don't lose it, too).

If you cannot find your cellphone, you can remotely activate the phone's video recorder and see what's in front of it to aid in your search. If it's dark (like in someone else's pocket or upside down) you can command the phone the flip over. If That doesn't work, press another button and it screams "LET ME GO! PUT ME DOWN! YOU'RE NOT MY DADDY!" in a child-like voice, just in case somebody stole it.

Note: simply turning off the phone will not help a thief, for this has a remote activation, as well. They will need to remove the battery, which will destroy the phone unless they have the key for it.